test: verify sync client teardown by owned resources

[Knucklessg1]

The successful synchronous-client close test compared process-wide descriptor, socket, selector and thread totals. Unrelated event-loop cleanup could therefore fail an otherwise correct teardown; this caused the root Python job failure in run 37716787573.

Assert that each client's owned loop is closed and its thread has terminated. Keep the 32 repeated-connect/close cycles and exactly-once transport-close assertion. The retry test now proves its thread remains alive after the deliberately failed first stop and terminates after retry. Remove the unused process-count helper and obsolete Linux-only skips; these checks use portable owned objects.

The successful-close test deliberately closes an unrelated event loop during its first cycle, with subsequent cycles exercising stable-resource repeated close. This extends EG-CONTRACT-R021's owned-resource proof to the successful/retried close paths; no whole-spec or release acceptance is claimed.

Validation against b4e272d:

• Baseline lifecycle suite: 5 passed.
• Controlled pre-fix reproduction: stable case passes, unrelated-cleanup case fails because totals decrease from (16 FDs, 2 sockets, 1 selector) to (13, 0, 0).
• Final lifecycle suite: 5 passed (0.21s).
• In-memory negative controls: skipping teardown, leaving the selector open, and duplicate transport close each fail the repaired assertion. Actual test resources are cleaned afterward; no production code changed.
• Broader lifecycle plus transport suite: baseline and candidate each report 25 passed, 13 failed. The same 13 declared tests fail because this source-only environment lacks the current native signing codec. No new failures; no native build requested or claimed.
• CCCC differential: no new/worsened complexity. Dupehound has no changed production source; KISS has no changed Rust source.
• All normal commit-stage checks passed, including Ruff, formatting, mypy, architecture, sanitizer and tracked privacy.
• Combined-diff jscpd: 2,683 existing pairs before and after, zero new pairs.
• All normal pre-push checks passed; no hook bypass.
• Exact candidate: a9780ff.

Only tests/test_sync_client_lifecycle.py changes. No test suppression, hook bypass, native/release gate change or deployment. Merge remains parent-coordinated; run 37716787573 is FAILED, not qualified. Privacy PR71 is unchanged.